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J. Exp. Med. Vol. 208 No. 6 1163-1177
Invariant NK T cells (iNKT cells) recognize
microbial and endogenous cellular lipid anti-
gens presented by CD1d molecules (Brigl and
Brenner, 2004; Kronenberg, 2005; Bendelac
et al., 2007). iNKT cells express an invariant
TCR- chain (V14J18 in mice and V24J18
in humans) paired with a limited set of TCR-
chains, display surface receptors typically found
on NK cells, and have a memory/effector phe-
notype in the absence of prior stimulation
(Godfrey et al., 2004). iNKT cells constitutively
express mRNA, but not protein, for IFN-,
poising them for rapid effector function (Stetson
et al., 2003). Together, these features distin-
guish iNKT cells from MHC-restricted T cells
and suggest distinct modalities of activation.
A growing body of evidence documents a criti-
cal role for iNKT cells during bacterial, viral,
fungal, and protozoan infections (Tupin et al.,
2007; Cohen et al., 2009). The protective, and
in some instances detrimental, functions of
iNKT cells during infection are often the re-
sult of their ability to rapidly produce copious
amounts of IFN- and to contribute to the re-
cruitment and activation of other cell types,
including neutrophils, macrophages, DCs, NK
cells, and B cells. These properties enable iNKT
cells to orchestrate and amplify the protective im-
mune response to infection (Brigl and Brenner,
2004; Tupin et al., 2007; Cohen et al., 2009).
Yet how CD1d-restricted iNKT cells, with their
limited TCR diversity, become activated rapidly
Michael B. Brenner:
Abbreviations used: GalCer,
GlcDAG, galactosyl GlcDAG;
erol; GSL, glycosphingolipid;
iNKT cell, invariant NK T cell;
MFI, mean fluorescence inten-
sity; TLR, toll-like receptor.
Elizabeth A. Leadbetter’s present address is Trudeau Institute,
Saranac Lake, NY 12983.
R.V.V. Tatituri and G.F.M. Watts contributed equally
to this paper.
Innate and cytokine-driven signals, rather than
microbial antigens, dominate in natural killer
T cell activation during microbial infection
Manfred Brigl,1,2 Raju V .V . Tatituri,2 Gerald F.M. Watts,2 Veemal Bhowruth,3
Elizabeth A. Leadbetter,2 Nathaniel Barton,2 Nadia R. Cohen,2 Fong-Fu Hsu,4
Gurdyal S. Besra,3 and Michael B. Brenner2
1Department of Pathology and 2Department of Medicine, Division of Rheumatology, Immunology, and Allergy,
Brigham and Women’s Hospital, Harvard Medical School, Boston, MA 02115
3School of Biosciences, University of Birmingham, Birmingham B15 2TT, England, UK
4Division of Endocrinology, Metabolism and Lipid Research, Washington University, St. Louis, MO 63110
Invariant natural killer T cells (iNKT cells) are critical for host defense against a variety of
microbial pathogens. However, the central question of how iNKT cells are activated by mi-
crobes has not been fully explained. The example of adaptive MHC-restricted T cells, studies
using synthetic pharmacological -galactosylceramides, and the recent discovery of microbial
iNKT cell ligands have all suggested that recognition of foreign lipid antigens is the main
driver for iNKT cell activation during infection. However, when we compared the role of
microbial antigens versus innate cytokine-driven mechanisms, we found that iNKT cell
interferon- production after in vitro stimulation or infection with diverse bacteria over-
whelmingly depended on toll-like receptor–driven IL-12. Importantly, activation of iNKT cells
in vivo during infection with Sphingomonas yanoikuyae or Streptococcus pneumoniae, patho-
gens which are known to express iNKT cell antigens and which require iNKT cells for effective
protection, also predominantly depended on IL-12. Constitutive expression of high levels of
IL-12 receptor by iNKT cells enabled instant IL-12–induced STAT4 activation, demonstrating
that among T cells, iNKT cells are uniquely equipped for immediate, cytokine-driven activa-
tion. These findings reveal that innate and cytokine-driven signals, rather than cognate
microbial antigen, dominate in iNKT cell activation during microbial infections.
© 2011 Brigl et al. This article is distributed under the terms of an Attribution–
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The Journal of Experimental Medicine
Innate iNKT cell activation during infection | Brigl et al.
NK cells and the systemic release of IFN- after iNKT
cell stimulation (Kitamura et al., 1999; Kawakami et al.,
2001; Matsuda et al., 2003).
In contrast to the TCR-mediated recognition of
microbial lipid antigens, iNKT cells can be activated
fully in response to microbial products by an innate
cytokine- and self-antigen–driven pathway. In this
scenario, iNKT cell activation results from combined
stimulation with a weak TCR-mediated signal from
recognition of endogenous CD1d-presented lipids,
together with cytokine-mediated co-stimulation by IL-12, re-
leased by DCs after toll-like receptor (TLR)–mediated activa-
tion (Brigl et al., 2003; Mattner et al., 2005; Nagarajan and
Kronenberg, 2007). iNKT cell activation after stimulation of
DCs with TLR agonists can be modulated by alterations in
CD1d-presented self-lipids and changes in CD1d expression
levels (Sköld et al., 2005; Raghuraman et al., 2006; Paget et al.,
2007; Salio et al., 2007). In some cases, such as stimulation with
LPS from Escherichia coli or during viral infection, iNKT cell
activation can be so dominantly driven by IL-12 and IL-18
that very little or no TCR-mediated stimulation by CD1d-
presented self-lipids is needed (Nagarajan and Kronenberg,
2007; Tyznik et al., 2008; Wesley et al., 2008). This innate
cytokine-driven pathway of activation allows iNKT cell recog-
nition of pathogens that express TLR ligands but appear to lack
CD1d-presented lipid antigens, such as viruses or the Gram-
negative bacterium Salmonella typhimurium (Brigl et al., 2003;
Mattner et al., 2005; Tyznik et al., 2008; Wesley et al., 2008).
The current model suggests that, dependent on the expres-
sion of antigens by the microbe, iNKT cell activation during
microbial infection is cognate, foreign antigen driven, or in-
nate cytokine driven (Mattner et al., 2005; Tupin et al., 2007;
in response to vastly diverse microbial infections remains in-
Two distinct pathways have been described for iNKT cell
activation during microbial infection. The microbial antigen-
driven pathway involves direct recognition of CD1d-presented
microbial lipid antigens by iNKT cells. Glycosphingolipids
(GSLs) found in Sphingomonas spp. (Kinjo et al., 2005; Mattner
et al., 2005; Sriram et al., 2005) and diacylglycerols isolated from
both Borrelia burgdorferi (Kinjo et al., 2006) and Streptococcus pneu-
moniae (Burrows et al., 2009) can be presented by CD1d mole-
cules, and these microbial lipid antigens have been proposed to
drive iNKT cell activation in a TCR-dependent manner during
infection (Kinjo et al., 2005, 2006; Mattner et al., 2005). After ex-
posure to these microbial antigens, iNKT cells produce both
IFN- and IL-4 within hours (Kinjo et al., 2005, 2006; Mattner
et al., 2005). The recognition of microbial glycosylceramides by
iNKT cells has been proposed to fill a gap in the innate recogni-
tion of Gram-negative LPS-negative -proteobacteria such as
Sphingomonas spp. and Ehrlichia spp. (Kinjo et al., 2005; Mattner
et al., 2005). The production of IFN- by iNKT cells in response
to antigen stimulation does not require IL-12 signaling; however,
IL-12 is known to play a critical role in the trans-activation of
Figure 1. Antigen- versus cytokine-driven pathways of
iNKT cell activation. (A–C) iNKT cell lines were cultured with
WT (filled squares) and CD1d-deficient (A), MyD88-deficient
(B), or IL-12p35–deficient (C; all open squares) BM-derived DCs
and stimulated with various concentrations of LPS, CpG, or
GSL-1 for 16–24 h. Cytokine concentrations in culture superna-
tants were measured by ELISA. Data are presented as means of
duplicate values ± SD and are representative of at least three
independent experiments. Bar graphs show percent inhibition
of IFN- secretion in culture supernatants comparing CD1d-
deficient (A), MyD88-deficient (B), or IL-12p35-deficient (C) DCs
(filled bars) to WT DCs after stimulation with 2 ng/ml LPS, 2 µg/ml
CpG, or 10 µg/ml GSL-1, and data are summarized from four
independent experiments (mean ± SD). Open bars in C repre-
sent the percentage of IFN- secretion in the presence of
10 µg/ml of blocking anti–IL-12 antibodies. Data are summarized
from two independent experiments. No significant inhibition
was observed with control antibodies (not depicted). (D and
E) Experiments showing IL-4 production in response to GSL-1,
LPS, or CpG using CD1d- (D) and MyD88- and IL-12–deficient
(E) BM DCs were performed as in A–C. IL-4 concentrations in
culture supernatants were measured by ELISA. n.a. indicates not
applicable because IL-4 levels with WT DCs were very low. Data
are presented as means of duplicate values ± SD and are repre-
sentative of at least three independent experiments.
JEM Vol. 208, No. 6
IFN- (Fig. 1 A, left). This IFN- response was dependent on
recognition of CD1d by iNKT cell lines, as use of CD1d-
deficient DCs resulted in markedly reduced IFN- secretion
(Fig. 1 A). To determine the requirement for TLR- and cyto-
kine-mediated stimulation for the activation of iNKT cell
lines, we performed experiments using DCs deficient in the
adaptor protein MyD88 or the production of IL-12, as well as
blocking antibodies against IL-12. Stimulation of iNKT cell
lines with GSL-1 was not altered when DCs deficient in
MyD88 were used (Fig. 1 B) and was only marginally re-
duced when IL-12–deficient DCs were used or when mAbs
to IL-12 were added to the cultures (Fig. 1 C). Thus, iNKT
cell activation by a microbial lipid antigen required CD1d
expression and was essentially independent of TLR signal-
ing or IL-12 production by APCs. The slightly reduced
iNKT cell IFN- response to GSL-1 in the presence of IL-12–
deficient DCs or after addition of mAb against IL-12 was
likely the result of a lack of IL-12–mediated amplification
of iNKT cell IFN- production that has been noted after
CD40L-mediated stimulation of DCs by antigen-activated
iNKT cells (Tomura et al., 1999).
In contrast to their direct stimulation by CD1d-presented
microbial lipid antigens, iNKT cells can also be stimulated
with TLR agonists in the presence of DCs by a self-antigen–
and cytokine-driven pathway that does not require the cognate
recognition of microbial antigens by iNKT cells (Brigl et al.,
2003; Nagarajan and Kronenberg, 2007; Paget et al., 2007;
Salio et al., 2007). Indeed, iNKT cell lines incubated with
WT DCs and stimulated with the TLR agonists LPS (TLR4)
or CpG (TLR9) produced copious amounts of IFN- (Fig. 1 A,
second and third panel). To determine the requirements for a
Brigl and Brenner, 2010). In this paper, we investigated the
relative contributions of microbial antigen– versus cytokine-
driven pathways in iNKT cell activation using a large panel
of diverse bacterial pathogens, several of which are known
to express iNKT cell antigens and/or have been shown to
require iNKT cells for protective immunity. Unexpectedly,
we found that iNKT cell IFN- production was dominantly
dependent on innate mechanisms with TLR-mediated sig-
naling and the production of IL-12 by APCs, irrespective of
whether or not bacteria express CD1d-presented iNKT cell
antigens. Furthermore, high levels of IL-12 receptor were ex-
pressed by iNKT cells, readying them for rapid cytokine-
mediated stimulation. Thus, our data suggest that innate
signals, together with cytokine-driven activation, are the
dominant pathway enabling rapid iNKT cell responses to di-
verse microbial infections.
Antigen- and cytokine-driven pathways of iNKT
Studies using only NKT cell hybridomas do not adequately
model the NKT cell activation mechanism that may occur in
vivo because such systems lack the potential to respond to
both antigen and cytokine signals. To investigate the mecha-
nisms of iNKT cell activation by microbes, we used a system
with primary mouse iNKT cell lines and BM-derived DCs
that is able to respond to a variety of stimuli (Chiba et al.,
2009). iNKT cell lines incubated with DCs and stimulated
with the CD1d-presented microbial GSL antigen GSL-1, which
is found in Sphingomonas spp. (Kinjo et al., 2005; Mattner
et al., 2005; Sriram et al., 2005), produced large amounts of
Table I. Bacteria used in this study
Organism Strain AntigensRole of iNKT in infectionReference
Burrows et al., 2009; Kawakami et al., 2003
Arrunategui-Correa and Kim, 2004
Brigl et al., 2003
Nieuwenhuis et al., 2002
Berntman et al., 2005; Brigl et al., 2003MT110
Induction of PBC
Mattner et al., 2005
Mattner et al., 2008
Kinjo et al., 2005
N40 BbGL-II (GalDAG)Protective Kinjo et al., 2006; Kumar et al., 2000; Tupin et al., 2008
Not protectiveBehar et al., 1999; Fischer et al., 2004
Description of bacteria used in this study, microbial lipid antigens, if any, described for the respective bacterium, and role of iNKT cells during infection.
GlcAGSL, glucuronic acid containing GSL; BbGL-II, B. burgdorferi glycolipid II; GalDAG, galactosyl diacylglycerol; PIM4, phosphatidylinositol mannoside 4; N.d., not described;
PBC, primary biliary cirrhosis.
aAmerican Type Culture Collection number.
bAntigenic activity of this lipid could not be confirmed by some investigators (Kinjo et al., 2006).
Innate iNKT cell activation during infection | Brigl et al.
DCs, TLR agonist–induced IFN- production by iNKT cell
lines was significantly reduced in response to LPS or CpG,
respectively, compared with stimulation in the presence of
WT DCs (Fig. 1 A). Because no exogenous microbial antigens
CD1d–TCR interaction, TLR signaling, and IL-12 stimula-
tion after activation of iNKT cells with TLR agonists, we
used DCs from CD1d-, MyD88-, or IL-12–deficient mice
and blocking antibodies against IL-12. Using CD1d-deficient
Figure 2. Detection of microbial lipid antigens expressed by bacteria. Lipids were extracted from bacteria and analyzed by ESI mass spectrometry.
(A–C) [M – H] adducts of the GSL-1 (GlcAGSL) antigen in S. capsulata, N. aromaticivorans, and S. yanoikuyae. (D) [M + CH3COO] adducts of the GalDAG
(BbGL-II) antigen in B. burgdorferi. (E) [M + Na]+ adducts of the GlcDAG and GalGlcDAG antigens in S. pneumoniae. For annotation of sphingosine and
acyl chain composition see Table S1. For corresponding LC and MS data of lipid antigens expressed by bacteria see SI Fig. 1.
JEM Vol. 208, No. 6
were present under these conditions, recognition of CD1d-
presented cellular self-antigens appeared to be required for
iNKT cell activation, as has been previously noted (Brigl
et al., 2003). When MyD88-deficient DCs were used during
stimulation of iNKT cell lines, IFN- production in response
to LPS or CpG was markedly reduced (Fig. 1 B). Further-
more, iNKT cell activation in response to LPS or CpG was
strikingly reduced when DCs deficient in IL-12 production
were used and when mAb to IL-12 was added to the cultures
(Fig. 1 C). In response to antigens, iNKT cells are also known
to secrete IL-4. After stimulation with GSL-1, iNKT cell
lines produced IL-4 in a CD1d-dependent and MyD88- and
IL-12–independent manner (Fig. 1, D and E). In re-
sponse to LPS or CpG, no significant amounts of
IL-4 were detected (Fig. 1, D and E). Thus, TLR
agonist–induced IFN- secretion by iNKT cell lines
in the presence of DCs was driven by recognition of
CD1d-presented self-antigens and IL-12, which are
produced by APCs after TLR-mediated activation,
whereas antigen-driven activation results in TLR-
and IL-12–independent IFN- and IL-4 secretion.
Expression of microbial iNKT cell antigens
To examine the mechanism of iNKT cell activation
in response to bacterial infection, we assembled a
panel of 11 bacteria, 5 of which are known to ex-
press CD1d-presented iNKT cell antigens (Table I).
The diverse panel of bacteria was selected to include
clinically important pathogens such as the Gram-
positive bacteria S. pneumoniae, Listeria monocytogenes
and Staphylococcus aureus, the Gram-negative bacteria
Pseudomonas aeruginosa, S. typhimurium, and E. coli,
the spirochete B. burgdorferi, the mycobacterium
Mycobacterium tuberculosis, and the -proteobacteria
Sphingomonas capsulata, Novosphingobium aromaticivorans,
and Sphingomonas yanoikuyae. Members of the class
of -proteobacteria are known to cause opportunis-
tic infections in humans and have been associated
with the induction of autoimmunity (Mohammed
and Mattner, 2009; Ryan and Adley, 2010). Further-
more, iNKT cells are critical in mice for protective
immunity to infection with S. pneumoniae, B. burgdorferi,
P. aeruginosa, S. capsulata, and L. monocytogenes, and
iNKT cells have been implicated in the develop-
ment of primary biliary cirrhosis in a mouse model
after infection with N. aromaticivorans (see Table I for
summary and references).
From among the panel of bacteria, microbial
CD1d-presented lipid antigens that are capable of stimulating
iNKT cells have been described for S. capsulata, N. aromaticivorans,
and S. yanoikuyae (all three GSL-1), B. burgdorferi (BbGL-II), and
S. pneumoniae (monoglycosyl- and diglycosyldiacylglycerol;
for details and references see Table I). To confirm that bacte-
ria used in this study indeed expressed iNKT cell antigens,
we isolated polar lipids from S. capsulata, N. aromaticivorans,
S. yanoikuyae, B. burgdorferi, and S. pneumoniae and sub-
jected the lipids to electrospray mass spectrometry analysis.
Lipids from S. capsulata, N. aromaticivorans, and S. yanoikuyae
yielded ions representing the [M – H] ions of the antigenic
-glucuronosylceramide GSL-1 (Fig. 2, A–C; and Table S1).
Figure 3. Cytokine responses of iNKT cells to diverse
bacteria. iNKT cell lines were cultured with WT (filled squares)
or CD1d-deficient (open squares) DCs in the presence of heat-
inactivated bacteria. (A and B) IFN- (A) and IL-4 (B) concen-
trations were measured in culture supernatants by ELISA after
16–24 h. Data represent means of duplicate values ± SD and
are representative of at least three independent experiments.
Innate iNKT cell activation during infection | Brigl et al.
Mechanism of bacteria-induced iNKT cell activation
We next examined the mechanism of iNKT cell activation in
response to the 11 bacteria described in the previous section.
iNKT cell lines incubated with WT DCs and stimulated with
any of the 11 heat-killed bacteria produced substantial
amounts of IFN- (Fig. 3 A). iNKT cell activation induced
by heat-killed bacteria required a CD1d-TCR interaction, as
IFN- production by iNKT cell lines in the presence of
CD1d-deficient DCs and any of the 11 heat-killed bacteria
was reduced by a mean of 50–80%, compared with stimula-
tion in the presence of WT DCs (Fig. 3 A). Little or no IL-4
was generated by iNKT cells after stimulation with most of
the heat-killed bacteria, whereas stimulation with B. burgdor-
feri, S. capsulata, N. aromaticivorans, and S. yanoikuyae resulted in
production of low amounts of IL-4 (Fig. 3 B and not depicted).
The amounts of IL-4 detectable in culture supernatants var-
ied substantially between experiments and were generally re-
duced after repeated stimulation of iNKT cell lines, and no
For B. burgdorferi lipids, ions corresponding to the [M +
CH3COO] adduct ions of -galactosyldiacylglycerol, the
BbGL-II antigen, were detected (Fig. 2 D and Table S1). In
positive-ion mode, the lipids from S. pneumoniae gave rise to
[M + Na]+ ions corresponding to the -glucosyldiacylglycerol
(GlcDAG), together with ions corresponding to the –galactosyl
GlcDAG (GalGlcDAG; Fig. 2 E and Table S1). The structural
assignments for individual lipid species were supported by
multiple-stage ion-trap tandem mass spectrometry (Table S1
and not depicted). Further analysis of the bacterial lipids by
liquid chromatography, light scattering detection, and mass
spectrometry analysis confirmed the presence of the respec-
tive antigenic lipid species in these five bacteria and suggested
that all bacteria expressed significant amounts of the respec-
tive antigens (Fig. S1). Thus, 5 of the 11 bacteria used in this
study expressed known iNKT cell lipid antigen species as ex-
pected. No lipids capable of stimulating iNKT cells directly
have so far been described for the other 6 bacteria.
Figure 4. Mechanism of iNKT cell activation in
response to diverse bacteria (A) iNKT cell lines were
cultured with WT or MyD88-deficient DCs in the
presence of heat-inactivated bacteria. Data for IFN-
secretion in culture supernatants are shown as per-
cent inhibition comparing MyD88-deficient to WT
DCs after stimulation with 10 bacteria per DC for
E. coli (Ec), P. aeruginosa (Pa), and S. typhimurium (St)
and with 100 bacteria per DC for S. pneumoniae (Sp),
L. monocytogenes (Lm), S. aureus (Sa), B. burgdorferi
(Bb), S. capsulata (Sc), N. aromoticivorans (Na),
S.yanoikuyae (Sy), and M. tuberculosis (Mt). Data are
summarized from four independent experiments
(mean ± SD). (B) Concentration of IL-12p40 (ng/ml,
filled bars) and IL-12p70 (pg/ml, open bars) measured
by ELISA in culture supernatants of DCs stimulated
with microbial products or heat-killed bacteria as in
Fig. 1 and Fig. 4 A, respectively (mean ± SD). No IL-18
was detected in culture supernatants and IFN- con-
centrations were not different between unstimulated
and stimulated conditions (not depicted). (C) iNKT cell
lines were cultured with WT or IL-12p35–deficient
DCs in the presence of stimuli as described in A. Data
are shown as percentage of IFN- secretion in cul-
ture supernatants comparing IL-12–deficient to
WT DCs after stimulation (filled bars; mean ± SD).
Open bars represent the percentage of IFN- secretion
in the presence of 10 µg/ml blocking mAb against IL-12.
No significant inhibition was observed with isotype-
matched control antibodies (not depicted). Data are
summarized from two independent experiments.
(D) Expression of the early activation marker CD25 by
iNKT cells after 24 h of co-culture with WT or IL-12–
deficient DCs in the presence of stimuli as in A. GSL-1
was used at 2 µg/ml, LPS at 10 ng/ml, and CpG at
2 µg/ml. (E) IL-4 production by iNKT cells in response
to heat-inactivated bacteria using WT (filled squares),
MyD88-deficient (open squares), or IL-12–deficient
(open circles) DCs. Experiments were performed as in
Fig. 1 (D and E). IL-4 concentrations in culture supernatants were measured by ELISA. Data are presented as means of duplicate values ± SD and are rep-
resentative of at least three independent experiments.
JEM Vol. 208, No. 6
obtained for the early activation marker CD69 (unpublished
data). iNKT cell IFN- production and CD25 expression
were also both reduced on day 2 after stimulation with anti-
gen, TLR agonists, or heat-killed bacteria, suggesting that
iNKT cell activation in the absence of IL-12 was not delayed
(Fig. S5, A and B). The weak IL-4 responses observed after
significant amounts of IL-4 were detected with iNKT cells
isolated directly from V14 TCR transgenic mice (unpub-
Next, we determined if iNKT cell activation by heat-
killed bacteria required TLR-mediated signaling by the APCs.
iNKT cell lines incubated with MyD88-deficient DCs plus
any of the 11 heat-killed bacteria resulted in reduced IFN-
secretion when compared with WT DCs (Fig. 4 A and Fig. S2).
Thus, whether the bacteria used for stimulation expressed
known microbial lipid antigens, iNKT cell IFN- secretion
was critically dependent on TLR-mediated signaling by DCs.
This result suggested that iNKT cell activation in response to
bacteria expressing microbial iNKT cell antigens might not
be mediated by TCR recognition of the microbial lipid anti-
gens and instead might be cytokine-driven.
IL-12, IL-18, and type I IFNs have been implicated in ac-
tivating iNKT cells after TLR-mediated stimulation of DCs
in the absence of iNKT cell recognition of CD1d-presented
microbial lipid antigens (Brigl et al., 2003; Mattner et al.,
2005; Nagarajan and Kronenberg, 2007; Paget et al., 2007;
Salio et al., 2007). We determined if any of these cytokines
were generated by DCs after stimulation with the panel of
heat-killed bacteria by measuring the concentration of IL-12,
IL-18, and IFN- in supernatants from DCs cultured with
TLR agonist, GSL-1, or any of the 11 heat-killed bacteria.
Significant quantities of IL-12p40 and IL-12p70, but not of
IL-18 or IFN-, were detected in supernatants of DCs exposed
to LPS, CpG, or any of the 11 heat-killed bacteria (Fig. 4 B
and not depicted). In contrast, no IL-12p40 or IL-12p70 was
detected in supernatants of DCs cultured in the presence of
the GSL-1 antigen. Then, we determined whether IL-12 was
required for IFN- secretion by iNKT cell lines in response
to stimulation with heat-killed bacteria. Indeed, incubation of
iNKT cell lines with IL-12–deficient DCs in the presence of
any of the 11 heat-killed bacteria resulted in substantially re-
duced IFN- production compared with stimulation in the
presence of WT DCs (Fig. 4 C and Fig. S2). Similarly, addition
of blocking mAb against IL-12 reduced iNKT cell IFN- se-
cretion after stimulation with any of the 11 heat-killed bacte-
ria (Fig. 4 C). In contrast, stimulation of iNKT cell lines with
IL-18–deficient DCs or after stimulation of iNKT cell/DC
co-cultures in the presence of antibodies blocking type I IFN
signaling did not show a requirement for IL-18 or type I
IFNs, respectively, in response to any of the 11 heat-killed
bacteria (Fig. S3, A and B). As described, MyD88- and IL-12–
deficient DCs were capable of presenting the purified GSL-1
antigen to stimulate iNKT cell lines but were not capable of
eliciting IFN- secretion by iNKT cell lines in response to
the TLR agonists LPS and CpG (Fig. 1). We next determined
if expression of the early activation marker CD25 on iNKT
cells after stimulation was dependent on IL-12. Expression
levels of CD25 were increased after stimulation with GSL-1,
TLR agonists, or heat-killed bacteria (Fig. 4 D and Fig. S4).
Stimulation with TLR agonists or heat-killed bacteria, but
not with GSL-1, resulted in decreased expression on CD25
when DCs deficient in IL-12 were used. Similar results were
Figure 5. iNKT cell response to live in vitro infection. DCs were in-
cubated with 2 µg/ml CpG, 10 µg/ml GSL-1, P. aeruginosas, S. typhimurium
(both 10 live bacteria per DC), L. monocytogenes, B. burgdorferi, S. capsu-
lata, or N. aromaticivorans (all 100 live bacteria per DC) for 3 h, followed
by washing and plating with iNKT cell lines. (A and B) IFN- (A) or IL-4
(B) concentrations were measured in culture supernatants by ELISA after
16–24 h. Data are displayed as means of triplicate measurements ± SD and
are representative of at least two independent experiments.
Innate iNKT cell activation during infection | Brigl et al.
irrespective of the expression of iNKT cell antigens by the
bacteria. This suggested that iNKT cell activation in response
to infected DCs was dominantly cytokine driven and that
TLR-mediated signals were required, whereas CD1d-mediated
signals played a variable role.
In vivo activation of iNKT cells
during Sphingomonas infection
Next, we analyzed iNKT cell activation in a mouse model of
Sphingomonas infection. Mononuclear cells were isolated from
stimulation with Borrelia and Sphingomonas spp. showed a
trend toward being reduced when MyD88-deficient DCs
were used and toward being increased when IL-12–deficient
DCs were used (Fig. 4 E). iNKT cells freshly isolated from
WT mice by tetramer sorting also showed a strong depen-
dence of the iNKT cell IFN- production on MyD88 and
IL-12, whereas little or no IL-4 was generated (Fig. S6, A and B).
The CD1d-dependence of TLR agonist- and bacteria-
induced activation of freshly isolated iNKT cell was reduced
compared with what was observed using iNKT cell lines,
which is likely the result of stimulation of iNKT cells using
TCR- antibodies and tetramer for their purification by cell
sorting, as has been observed previously (Matsuda et al., 2003;
Nagarajan and Kronenberg, 2007).
To determine if alterations in CD1d expression levels by
the APCs may contribute to the observed iNKT cell activa-
tion, CD1d expression levels were determined on WT DCs
after exposure to heat-killed bacteria. Stimulation with the
TLR agonists LPS and CpG and all 11 of the heat-killed bac-
teria, but not GSL-1, resulted in increased expression of CD1d
by DCs (Fig. S7, A and B). The increased expression of
CD1d after stimulation was absent in the presence of MyD88-
deficient DCs, suggesting that TLR-mediated signaling was
required. Thus, iNKT cell activation by a wide variety of
heat-killed bacterial pathogens in vitro, including bacteria
which expressed CD1d-presented microbial lipid antigens,
such as Sphingomonas spp., B. burgdorferi, M. tuberculosis, or S.
pneumoniae, was dominantly dependent on TLR signaling and
iNKT cell activation after live in vitro infection
To further investigate the mechanism of iNKT cell activation
in response to DCs infected with live bacteria, we used an
in vitro live infection model with three microorganisms shown
to express iNKT cell antigens (B. burgdorferi, S. capsulata, and
N. aromaticivorans; Table I) and three microorganisms for which
iNKT cell antigens have not been described (P. aeruginosa,
S. typhimurium, and L. monocytogenes). DCs were infected with
bacteria for 3 h and infected DCs were then co-cultured with
iNKT cell lines, resulting in production of substantial amounts
of IFN- for all six bacteria tested (Fig. 5 A). To determine
the relative requirements for a CD1d–TCR interaction, TLR
signaling, and IL-12 stimulation after activation of iNKT cells
with infected DCs, we used DCs from CD1d-, MyD88-, or
IL-12–deficient mice. Infection of DCs deficient in CD1d,
MyD88, or IL-12 production with P. aeruginosa, S. typhimurium,
L. monocytogenes, B. burgdorferi, S. capsulata, or N. aromaticivorans
resulted in reduced IFN- production by iNKT cell lines,
compared with infection of WT DCs (Fig. 5 A). Live in vitro
infection with B. burgdorferi, and to a lesser extent S. capsulata,
resulted in the production of IL-4 (Fig. 5 B). This IL-4 pro-
duction was largely MyD88 dependent but IL-12 indepen-
dent. Thus, similar to the results obtained after stimulation
with heat-killed bacteria, iNKT cell IFN- production in re-
sponse to DCs infected with live bacteria was dependent
on CD1d recognition, TLR-signaling, and IL-12 stimulation,
Figure 6. In vivo activation of iNKT cells during S. yanoikuyae
infection. (A) CD69 surface staining on CD1d tetramer–positive iNKT cells
isolated from the livers of uninfected WT mice (dotted line) or 18 h after i.v.
infection with S. yanoikuyae (bold line). (B) IFN- secretion by CD1d tetra-
mer–positive liver iNKT cells from uninfected animals (left, bold line) or 18 h
after i.v. infection with S. yanoikuyae (right, bold line) in comparison with
staining with isotype control antibodies (dotted line). (C and D) Comparison
of IFN- secretion (C) and CD69 expression (D) by CD1d tetramer–positive
liver iNKT cells in WT and IL-12–deficient mice 18 h after i.v. infection with
S. yanoikuyae (mean ± SD). (E) WT or IL-12-deficient mice were injected i.v.
with 20, 5, or 1.25 µg GSL-1, and IFN- secretion by CD1d tetramer–
positive liver iNKT cells was determined after 80 min. Means ± SD are
shown. Data are pooled from three independent experiments and represent
four to six mice per condition. (F) IL-4 secretion by liver iNKT cells 45 min
after injection of 2 µg -GalCer. Similar results were observed with GSL-1
(not depicted). (G) IL-4 secretion by liver iNKT cells from uninfected animals
or 18 h after i.v. infection with S. yanoikuyae in comparison with staining
with isotype control antibodies. Data are representative of two independent
experiments using three to four mice per condition.
JEM Vol. 208, No. 6
Thus, iNKT cells became activated rapidly after S. pneumoniae
infection. Cytokine secretion assays showed significant IFN-
secretion by iNKT cells on days 2 and 4 after infection that
returned to baseline by day 7 after infection (Fig. 7 D). No
significant production of IL-4 by iNKT cells was observed in
either WT or IL-12–deficient mice during infection at any of
the time points examined (Fig. 7 D and not depicted). Thus,
iNKT cells became activated rapidly and expressed IFN-
early during pulmonary infection with S. pneumoniae.
Next, we determined whether IL-12 was required for the
activation of iNKT cells during S. pneumoniae infection, as
was the case in the in vitro experiments described in the pre-
vious paragraphs. IL-12 deficiency resulted in reduced IFN-
secretion and CD69 expression on days 2 and 4 after infection
WT mice 18 h after intravenous infection with S. yanoikuyae
and iNKT cells were identified using CD1d tetramers. CD1d
tetramer–positive iNKT cells from livers of uninfected mice
stained at intermediate intensity for the early activation
marker CD69 (mean fluorescence intensity [MFI], 465 ± 13)
and, 18 h after infection, expression of CD69 increased to an
MFI of 2,990 ± 639 (Fig. 6 A). IFN- secretion by iNKT
cells showed that a mean of 41 ± 14% of the CD1d tetramer–
positive iNKT cells were positive 18 h after infection, compared
with a mean of 2 ± 0.1% in uninfected controls (Fig. 6 B).
Thus, iNKT cells became rapidly activated after S. yanoikuyae
infection. To assess whether IL-12 was required for the in vivo
activation of iNKT cells during S. yanoikuyae infection, as
suggested by our in vitro experiments, we compared IFN-
secretion and CD69 expression by liver iNKT cells in WT
versus IL-12–deficient mice after infection. 18 h after infec-
tion, IL-12 deficiency resulted in reduced IFN- secretion
and CD69 expression (Fig. 6, C and D).
To compare the requirement for IL-12 in iNKT cell IFN-
secretion after infection with S. yanoikuyae bacteria that
express the GSL-1 antigen with iNKT cell activation induced
by the GSL-1 antigen, we analyzed iNKT cell activation after
stimulation with purified GSL-1 antigen. After injection of a
range of doses of GSL-1 antigen, IFN- secretion by CD1d
tetramer–positive liver iNKT cells was similar in WT or
IL-12–deficient mice (Fig. 6 E). IL-4 production by iNKT
cell in vivo was readily observed after injection of the
-galactosylceramide (GalCer) antigen; however, during infec-
tion with S. yanoikuyae, S. capsulata, or N. aromaticivorans no sig-
nificant amounts of IL-4 were detected in either WT or
IL-12–deficient mice (Fig. 6, F and G; and not depicted). Thus,
iNKT cell activation after infection with a GSL-1–expressing
microbe is dependent on IL-12, whereas iNKT cell activation
with a microbial antigen alone does not require IL-12.
iNKT cell activation during S. pneumoniae infection
Next, we examined the role of iNKT cells after infection
with S. pneumoniae. Infection of WT and iNKT cell–deficient
J18/ mice with S. pneumoniae showed reduced survival of
iNKT cell–deficient J18/ mice (Fig. 7 A). Between days 6
and 12 after infection, >50% of J18/ mice succumbed to
infection, whereas none of the WT animals died. Similar re-
sults were obtained in CD1d-deficient animals (Fig. S8 A).
Bacterial burden in lungs of infected WT or J18/ animals
were comparable on day 3 after infection. However, on
day 6 after infection, bacterial burden in lungs of J18/
were 1,000-fold higher when compared with WT animals
(Fig. 7 B). Thus, iNKT cells were critically important for protec-
tive immunity after pulmonary infection with S. pneumoniae.
To examine iNKT cell activation after pulmonary infec-
tion with S. pneumoniae, lymphocytes were isolated from lungs
of infected and uninfected animals and iNKT cells were ana-
lyzed by flow cytometry for surface expression of the activa-
tion marker CD69 and for secretion of IFN-. Expression of
CD69 was intermediate on iNKT cells from uninfected mice
and increased on days 2, 4, and 7 after infection (Fig. 7 C).
Figure 7. In vivo activation of iNKT cells during S. pneumoniae in-
fection. WT or J18-deficient mice were infected intranasally with S. pneu-
moniae. (A) Survival was recorded daily for 2 wk for WT (filled squares;
n = 9) and J18/ (open squares; n = 9) mice. Results are representative of
two independent experiments. (B) The number of CFU was determined in
lung tissues of WT (open bars; n = 6) or J18/ (filled bars; n = 6) on days 3
and day 6 after infection. Data are pooled from two independent experi-
ments (mean ± SD). (C and D) Lymphocytes were isolated from lungs of
mice infected intratracheally with S. pneumoniae or from uninfected mice
and analyzed by flow cytometry. Staining for surface expression of CD69
(C) or secreted IFN- or IL-4 (D) on CD1d tetramer–positive lymphocytes are
shown. Data represent means ± SD (n = 6) and are pooled from two inde-
pendent experiments. (E and F) WT or IL-12p35–deficient mice were in-
fected intratracheally with S. pneumoniae and secretion of IFN- (E) or
expression of CD69 by iNKT cells (F) was determined as described in C and
D on day 2 after infection. Data represent means ± SD for three to four
mice per group and one experiment of two similar experiments is shown.
Innate iNKT cell activation during infection | Brigl et al.
In comparison, much higher levels of IL-12R2
mRNA were detected in NK cells. Interme-
diate levels of IL-12R2 were expressed by
CD4+ and CD4 iNKT cells isolated from spleen, and high
levels, similar to those detected on NK cells, were detected in
CD4+ and CD4 liver iNKT cells. Second, we determined if
the expression levels of IL-12R mRNA correlated with pro-
tein expression. Using CD1d tetramers to detect iNKT cells,
we found that expression of IL-12R1 could be detected ex-
clusively on tetramer+/CD3+ lymphocytes (iNKT cells) but
not on tetramer/CD3 lymphocytes (mainly NK cells) or
tetramer/CD3+ (containing MHC-restricted CD4 and CD8
T cells and T cells; Fig. 8 B).
The specific cellular effects of IL-12 are mainly a result of
its ability to induce STAT4 activation. To determine if the
constitutive expression of high levels of IL-12 receptor en-
abled prompt STAT4 activation in iNKT cells after IL-12
stimulation, we purified naive and memory CD4 T cells, NK
cells, and iNKT cells by cell sorting and stimulated the puri-
fied cell populations with IL-12 for 1 h. Intracellular phos-
phorylation of STAT4 was assessed by antibody staining and
flow cytometry. No STAT4 phosphorylation was detected in
naive and memory CD4 T cells (Fig. 8 C). In contrast, phos-
phorylation of STAT4 was detected in a large number of NK
cells and iNKT cells. Thus, unique among T cells but similar
(Fig. 7, E and F; and Fig. S8, B and C). Thus, most of the
iNKT cell activation and IFN- production during S. pneu-
moniae infection in vivo was critically dependent on IL-12.
Expression and function of IL-12 receptor on iNKT cells
Responsiveness to IL-12 correlates with expression of IL-12
receptor (IL-12R) on NK and activated T cells (Presky et al.,
1996; Trinchieri, 2003). To determine the expression of IL-12R
by iNKT cells, we took complementary approaches. First, ex-
pression of mRNA for the two IL-12R subunits, IL-12R1
and IL-12R2, was determined by microarray analysis in
iNKT cells and other lymphocyte subsets. IL-12R1, which
is responsible for binding IL-12, was expressed at very low
levels by naive CD4 and CD8 T cells, memory CD8 T cells,
T cells, and NK cells, whereas slightly higher levels were
detected in memory CD4 T cells (Fig. 8 A). In contrast, much
higher levels of IL-12R1 mRNA were detected in both
CD4+ and CD4 iNKT cells isolated from spleen or liver.
IL-12R2, which is required for IL-12R–mediated signaling,
was expressed at very low levels by naive CD4 and CD8 T cells,
memory CD8 T cells, and T cells, whereas slightly higher
levels were expressed by memory CD4 T cells (Fig. 8 A).
Figure 8. Expression and function of IL-12 recep-
tor on iNKT cells. (A) IL-12R1 (left) and IL-12R2
(right) mRNA expression in purified splenocyte subsets
or iNKT cell subsets purified from spleen or liver was
assessed by microarray. Data are representative of two
to four independent experiments and are shown as
mean ± SD. (B) Flow cytometry staining of CD19 sple-
nocytes with CD1d tetramers and anti-CD3 antibodies
(left). The tetramer/CD3 gate contains NK cells, the
tetramer/CD3+ gate contains MHC-restricted CD4 and
CD8 T cells and T cells, and the tetramer+/CD3+
gate contains iNKT cells. Surface expression of IL-12R1
was determined by flow cytometry on gated lympho-
cyte subpopulations. (C) Naive and memory CD4 T cells,
NK cells, and iNKT cells were isolated from spleens of
WT mice and purified by cell sorting. Cells were stimu-
lated in the presence of 1 ng/ml recombinant IL-12 for
1 h and subsequently stained with antibodies against
the phosphorylated form of STAT4 (solid lines) or iso-
type control antibodies (dotted lines). (D) iNKT cell lines
were incubated with WT (filled squares) or CD1d/
(open squares) DCs in the presence of various concen-
trations of recombinant IL-12. Data are presented as
means of duplicate values ± SD and are representative
of at least three independent experiments. (E) iNKT cell
lines were incubated with WT DCs and various concen-
trations of the microbial lipid antigen GSL-1 in the ab-
sence or presence of various concentrations of
recombinant IL-12. IFN- concentrations were mea-
sured in culture supernatants by ELISA after 16–24 h.
Data are presented as means of triplicate values ± SD
and are representative of two independent experiments.
JEM Vol. 208, No. 6
iNKT cell activation and IFN- secretion in response to all
microbes tested. Underscoring the critical and dominant role
of IL-12 for rapid activation of iNKT cells, we detected
that constitutive expression of high levels of IL-12R1 and
IL-12R2 chains by iNKT cells and IL-12 stimulation rapidly
led to STAT4 activation in iNKT cells. In contrast, MHC-
restricted T cells lack constitutive expression of components
of the IL-12 receptor and are only known to express func-
tional IL-12 receptor after activation and Th1 differentiation
(Presky et al., 1996; Trinchieri, 2003). In agreement with pre-
vious studies, we found that naive and memory CD4+ T cells
were not able to activate STAT4 after IL-12 stimulation in
the absence of additional stimulation or differentiation.
Recently, MAIT (mucosal-associated invariant T) lymphocytes,
another lymphocyte population characterized by expression
of an invariant TCR- chain, have been shown to become
activated by microbes independently of TLR- and cytokine-
mediated stimulation, suggesting that these innate T cells require
recognition of cognate microbial antigens for their activation
(Le Bourhis et al., 2010). Thus, among T cells, iNKT cells
to NK cells, high constitutive expression of functional IL-12
receptor appears to provide a mechanistic explanation for the
ability of iNKT cells to rapidly secrete IFN- in response to
Last, we sought to address how IL-12–mediated co-
stimulation alters iNKT cell activation by CD1d-presented
self- and microbial lipids. The addition of recombinant IL-12
to iNKT cell lines cultured in the presence of WT DCs re-
sulted in notably increased IFN- secretion, as compared
with the addition of IL-12 to iNKT cell lines co-cultured in
the presence of CD1d-deficient DCs (Fig. 8 D). As no exog-
enous antigens were added in this system, the stimulation pro-
vided by CD1d recognition was likely a result of recognition
of CD1d-presented self-lipids. Next, we determined how co-
stimulation with IL-12 altered iNKT cell responses to micro-
bial antigens. A cross-titration of GSL-1 and IL-12 revealed
that in particular low concentrations of GLS-1 and low con-
centrations of IL-12 resulted in a synergistic stimulation of
IFN- by iNKT cells (Fig. 8 E). Thus, weak TCR-mediated
stimulation of iNKT cells with CD1d-present self- or micro-
bial antigens was significantly amplified by IL-12, resulting in
potent secretion of IFN- by iNKT cells.
iNKT cells have been shown to play important protective
roles during several bacterial, parasitic, and viral infections.
However, the central question of how they are activated by
microbes is not fully explained. Because viruses lack lipid an-
tigens and antigens have only been described for a few patho-
gens so far, it is important to determine how many diverse
pathogens can activate iNKT cells. To investigate the mecha-
nism of iNKT cell activation in response to infection, we used
a large panel of diverse bacteria, including several that express
known CD1d-presented iNKT cell lipid antigens. Unexpect-
edly, we found that, irrespective of the expression of CD1d-
presented lipid antigens by the microbes, microbe-induced
iNKT cell activation and IFN- production in vitro and in
vivo was critically dependent on IL-12 released by DCs after
TLR-mediated activation. Thus, our data do not support a
recently proposed model in which iNKT cell activation dur-
ing infection is driven by TCR-mediated recognition of
CD1d-presented microbial antigens (Mattner et al., 2005;
Tupin et al., 2007). Instead, our data suggest that innate cyto-
kine-driven activation is the dominant pathway for iNKT
cell activation in response to virtually all infectious agents ex-
amined to date that induce the production of IL-12 by APCs
after TLR-mediated activation (Fig. 9).
In addition to IL-12 as a critical mediator for stimulating
iNKT cell responses to microbes, other studies have suggested
a role for IL-18 and type I IFN in the iNKT cell response to
E. coli LPS or the TLR9 agonist CpG, respectively (Nagarajan
and Kronenberg, 2007; Paget et al., 2007). However, our in
vitro experiments did not show a significant role for IL-18 or
type I IFNs in response to any of the 11 bacterial pathogens
tested. Instead, our in vitro and in vivo experiments repeat-
edly implicated IL-12 as the critical cytokine required for
Figure 9. Innate and cytokine-driven iNKT cell activation during
microbial infection. iNKT cell activation during microbial infection is
dominantly driven by innate TLR-mediated signals and IL-12, which is
released by DCs after stimulation with microbial products. In addition,
TCR-mediated stimulation contributes to iNKT cell activation. However,
the TCR-mediated signal alone, provided either by recognition of CD1d-
presented self- or microbial antigens, is not sufficient to result in iNKT cell
IFN- production in the absence of IL-12 stimulation. The constitutive
expression of high levels of IL-12 receptor endows iNKT cells with the
ability to respond rapidly to cytokine-mediated stimulation and ensures
immediate iNKT cell activation in response to virtually any infectious
agent that induces the production of IL-12, irrespective of the expression
of microbial lipid antigens, thus allowing iNKT cells to overcome their
restricted TCR specificity.
Innate iNKT cell activation during infection | Brigl et al.
that induces IL-12 production, irrespective of the expression
of iNKT cell ligands, and thus allows iNKT cells to overcome
their restricted TCR repertoire and conserved mode of anti-
gen-recognition. Such a mechanism of activation may be
critical for iNKT cell responses to viral infection, because vi-
ruses are not known to encode enzymes for the production of
unique microbial lipids, and for responses to other micro-
organisms that lack expression of CD1d-presented iNKT cell
ligands. For the organisms that contain iNKT cell lipid anti-
gens, synthesis, uptake, and loading of antigens into CD1d
molecules, and subsequent transport of antigen–CD1d com-
plexes to the surface of APCs, may be inefficient and slower
than the IL-12–driven iNKT cell response.
The rapid innate cytokine-driven activation of iNKT
cells provides the evolving immune response with T cell ef-
fector functions early during the course of infection that
otherwise would only be available after the expansion and
differentiation of antigen-specific MHC-restricted T cells
(Brigl et al., 2003; Chiba et al., 2008). During infection, iNKT
cell–secreted IFN- appears to serve several critically impor-
tant functions. This includes a critical role in activation of
macrophages and in neutrophil-mediated clearance of patho-
gens (Nieuwenhuis et al., 2002; Nakamatsu et al., 2007). In addi-
tion, the early IFN- provided by iNKT cells has been shown
to be critical for the development of adaptive Th1 immune
responses and may affect the magnitude and quality of MHC-
restricted CD4 and CD8 T cell responses during infection
(Tupin et al., 2007; Cohen et al., 2009). Together, iNKT cells,
and the IFN- they produce, have emerged as critical regula-
tors of the early immune response during infection.
In summary, our findings indicate that during infection
with a variety of bacterial pathogens, including bacteria which
express known iNKT cell antigens, iNKT cell activation is
driven predominantly by IL-12, rather than by TCR-mediated
recognition of CD1d-presented microbial antigens (Fig. 9).
In contrast to the dominant role observed for foreign antigen
recognition in the activation of MHC-restricted T cells, the
lack of dependence on specific recognition of cognate micro-
bial antigen defines a different role for the TCR in innate
T cell activation. For the activation of iNKT cells during in-
fection, CD1d–TCR interactions instead appear to deter-
mine the cell–cell interaction between iNKT cells and APCs
and allow T cell activation to become manifest. This may con-
trol such innate cytokine-driven activation so that it is local-
ized to sites of APC-produced IL-12. The constitutive
expression of high levels of IL-12 receptor uniquely equips
iNKT cell for immediate cytokine-driven responses. Our
results suggest a unified dominantly innate cytokine-driven
model of iNKT cell activation that explains how these cells
become activated efficiently and rapidly in response to highly
diverse microbial pathogens.
MATERIALS AND METHODS
Mice. C57BL/6, IL-12p40/, IL-12p35/, and IL-18/ mice were ob-
tained from The Jackson Laboratory. J18/ (Cui et al., 1997) and CD1d/
mice (Exley et al., 2003) were provided by M. Exley (Harvard Medical
School, Boston, MA), V14J18 TCR transgenic mice (Bendelac et al., 1996)
appear to be uniquely equipped to promptly secrete IFN-
upon primary encounter of an inflammatory condition that
results in the production of IL-12.
Our in vitro experiments showed that full iNKT cell acti-
vation in response to all pathogens tested also required CD1d-
dependent signals and that increased expression of CD1d by
bacteria-stimulated APCs may contribute to iNKT cell acti-
vation. However, in the absence of co-stimulation by IL-12,
the CD1d-mediated signals were not sufficient to induce
IFN- secretion by iNKT cells. Our in vitro studies showed
that iNKT cell responses to weak self-antigens or to low con-
centrations of microbial antigen were amplified by exoge-
nously added IL-12. Therefore, although not able to activate
iNKT cells through TCR-mediated signaling alone, it seems
likely that even small numbers of CD1d–microbial antigen
complexes or presentation of more potent self-antigens may
contribute to or modulate iNKT cell activation during infec-
tion in the presence of IL-12–mediated co-stimulation. Fur-
thermore, the requirement for CD1d-TCR–mediated signals
may ensure that IL-12–amplified iNKT cell activation occurs
only in close contact with CD1d-expressing APCs such as
monocytes, DCs, macrophages, and B cells. The lack of de-
pendence on specific recognition of CD1d-presented micro-
bial antigens found in this study suggests that the exceptional
potency of the pharmacological iNKT cell agonist -GalCer,
which is much greater than that of any naturally occurring iNKT
cell antigen found to date, may have led to overestimation of the
importance of microbial antigens in iNKT cell activation.
iNKT cell responses to all of the microbial antigens de-
scribed so far result in the production of both IFN- and IL-4
by iNKT cells both in vitro and in vivo. Most of the bacteria
tested in our studies resulted in a bias toward production of
IFN-, using iNKT cell lines, freshly isolated iNKT cells, and
during in vivo infection. Interestingly, the occasionally ob-
served IL-4 induced by Sphingomonas spp. in vitro was in-
creased in the absence of IL-12, suggesting that IL-12 may
play an important role in polarizing iNKT cell responses to
infection. However, in vivo infection with Sphingomonas spp.
in IL-12–deficient mice did not result in a notable increase in
the number of iNKT cells producing IL-4 (unpublished data).
Stimulation of iNKT cells with B. burgdorferi resulted in IL-4
production by iNKT cells in our studies, which is consistent
with previous studies detecting IL-4 production by a small
number of iNKT cells in spleen after B. burgdorferi infection
(Tupin et al., 2008). The in vitro IL-4 production observed in
our studies appeared to be IL-12 independent but MyD88
dependent, suggesting that innate signals, rather than micro-
bial antigens, are responsible for the iNKT cell IL-4 produc-
tion in response to this bacterium. Further studies will be
required to understand the nature of the signals responsible
for bacteria-induced IL-4 production by iNKT cells.
Activation of iNKT cells during infection driven by in-
nate signals and cytokines, rather than microbial antigens,
provides several advantages. As an amplification loop for in-
nate signals, cytokine-mediated activation enables the activa-
tion of iNKT cells in response to virtually any microorganism
JEM Vol. 208, No. 6
leave a minimal residual abundance of precursor ion (around 10%). Mass spec-
tra were accumulated in the profile mode, typically for 3–10 min for MSn (n =
2, 3, and 4) spectra. The mass resolution of the instrument was tuned to 0.6 D
at half peak height. Synthetic GSL-1 was provided by the NIH tetramer facility
and P.B. Savage (Brigham Young University, Provo, UT; Mattner et al., 2005).
Synthesis of -GalCer has been previously described (Veerapen et al., 2009).
In vitro infections. Bacteria were grown to mid-log phase, as described in
Bacteria, and added to DCs cultured in antibiotic-free complete RPMI. After
a 3-h incubation at 37°C, infected DCs were washed 3× in complete RPMI.
Infected DCs (5 × 104/well) were co-cultured with iNKT cell lines (105/
well) in complete RPMI medium in 96-well plates for 16–24 h.
In vivo infection and flow cytometry. S. pneumoniae was grown as de-
scribed in Bacteria and mice were anesthetized with i.p. ketamine/xylazine
or by inhalation isoflurane and inoculated intranasally or intratracheally, re-
spectively, with 2–3 × 106 bacteria in PBS. CFUs were determined on blood
agar plates. For flow cytometric analysis, mononuclear cells were isolated
from lungs perfused with PBS followed by digestion with collagenase/DNase.
S. yanoikuyae was grown in TSB supplemented with 5% sheep blood and pas-
saged in SCID mice. Mice were injected i.v. with 4 × 109 bacteria in 200 µl
PBS. Cells were stained with antibodies against CD45, CD19, TCR-,
PBS57-loaded CD1d tetramers (NIH Tetramer Core Facility), CD69, or iso-
type-matched control antibodies. Secretion of IFN- or IL-4 was determined by
cytokine secretion assay (Miltenyi Biotec) according to the manufacturer’s in-
structions. Flow cytometric data were collected on a LSR II flow cytometer
(BD) and analyzed with FlowJo software (Tree Star).
IL-12 receptor expression. iNKT cells were purified from spleen or liver
on a MoFlo cell sorter (Dako) gating on CD19, B220, Ter119, CD11b,
CD11c, LyG/Gr-1, CD45+, TCRint, CD1d-tetramer+, CD4+, or CD4
iNKT cells, followed by RNA extraction with Trizol (Invitrogen), RNA ampli-
fied, and hybridized to Affymetrix 1.0 ST MuGene arrays within the NI-
AID/NIH ImmGen project (protocols and raw data available at http://www
.immgen.org; Heng and Painter, 2008). For flow cytometric analysis, cells
were stained with CD1d tetramers and antibodies against IL-12R1 (BD).
STAT4 phosphorylation. Naive (CD62LhiCD44lo) and memory
(CD62LloCD44hi) TCR+CD4+ T cells, NK cells (NK1.1+TCR-), and
iNKT cells (as in the previous section) were isolated by negative selection
and cell sorting. 1–5 × 105 purified cells were cultured in completed RPMI
and stimulated with recombinant IL-12 for 1 h at 37°C and subsequently stained
with antibodies against pSTAT4 (BD) according to manufacturer’s protocols.
Online supplemental material. Fig. S1 shows mass spectrometry analysis
of microbial lipids separated by liquid chromatography. Fig. S2 shows repre-
sentative dose titrations for bar graphs depicted in Fig. 4 (A and C). Fig. S3
shows IL-18 and type I IFN independence of iNKT cell activation in response
to microbes. Fig. S4 shows representative FACS plots for bar graphs depicted
in Fig. 4 D. Fig. S5 shows IL-12 dependence of IFN- secretion and CD25
expression by iNKT cells on day 2 after activation with microbial products.
Fig. S6 shows cytokine responses to microbial stimulation of freshly isolated
iNKT cells. Fig. S7 shows MyD88-dependent CD1d expression by DCs after
stimulation with microbial products. Fig. S8 shows survival of WT and CD1d-
deficient mice after S. pneumoniae infection and in vivo activation of iNKT
cells on day 4 after S. pneumoniae infection. Online supplemental material is
available at http://www.jem.org/cgi/content/full/jem.20102555/DC1.
The authors would like to thank the NIH Tetramer Core Facility for mouse CD1d
tetramer and GSL-1, A. Bendelac, M. Exley, and K. Kobayashi for providing mice,
K. Kawakami, L. Bockenstedt, and R. Blumberg for providing bacteria, P.B. Savage for
providing GSL-1, P. Brennan for critically reading the manuscript, and S. Moskowitz
for help with the artwork.
This work was supported by NIH grants AI077795 (M. Brigl), AI028973, and
AI063428 (both M.B. Brenner) and grants P41-RR-00954, R37-DK-34388, and P60-
DK-20579 (Mass Spectrometry Center, Washington University).
The authors have no conflicting financial interests.
by A. Bendelac (University of Chicago, Chicago, IL), and MyD88/ (Adachi
et al., 1998) by K. Kobayashi (Harvard Medical School, Boston, MA). All
mice were on the C57BL/6 background, were housed in specific pathogen-
free conditions or a biosafety level 2 animal facility after in vivo infections,
and female mice at 6–14 wk of age were used for experiments. All animal
studies were approved by the Dana-Faber Cancer Institute Animal Care and
In vitro iNKT cell assay. iNKT cell lines were generated and maintained
in complete RPMI medium (RPMI supplemented with 10% FBS [Gemini],
Hepes [Invitrogen], L-glutamine, penicillin/streptomycin, and 2-ME) sup-
plemented with 20 U/ml of recombinant mouse IL-2 (PeproTech) and
10 ng/ml IL-7 as previously described (Chiba et al., 2009). DCs were generated
from BM of mice cultured in complete RPMI supplemented with 10 ng/ml
of recombinant mouse GM-CSF and 1 ng/ml IL-4 for 5–8 d and purified
with CD11c magnetic beads (Miltenyi Biotec). iNKT cell lines (105/well)
were cultured with DCs (2 × 104/well) in 96-well plates (Costar; Corning)
in complete RPMI. Freshly isolated iNKT cells were obtained from spleens
of WT B6 animals after depletion of CD19+ and CD8a+ cells using magnetic
beads (Miltenyi Biotec), followed by cell sorting using CD1d tetramers and
TCR- staining on a FACSAria IIu instrument (BD). GSL-1 (National In-
stitutes of Health [NIH] Tetramer Core Facility), LPS (Salmonella enterica
serotype Typhimurium; Sigma-Aldrich), CpG (ODN1826; InvivoGen),
recombinant mouse IL-12 (PeproTech), or bacteria were added and cultures
were incubated for 16–48 h at 37°C. Culture supernatants were analyzed by
ELISA with matched antibody pairs (IFN- and IL-4 [BD], sensitivity 10
and 2 pg/ml, respectively; IL-18 and IFN- [R&D Systems]). Blocking anti–
IL-12 (BD) and anti–mouse IFN-/ R1 antibodies (R&D Systems) were
added to cultures at 5–10 µg/ml. Expression of CD25 was determined on
iNKT cells by flow cytometry (gated by FSC/SSC). Surface expression of
CD1d on CD11c+ BM DCs was determined using CD1d Abs (BD) after in-
cubation with various stimuli.
Bacteria. S. pneumoniae (for strains and references see Table I; provided by
K. Kawakami (University of the Ryukyus, Nishihara, Okinawa, Japan), cultured
in THY [BD] or BHI [Oxoid] medium), L. monocytogenes (BHI medium),
S. aureus (LB medium; BD), P. aeruginosa (LB medium, provided by R. Blum-
berg, Harvard Medical School, Boston, MA), S. typhimurium (LB medium),
E. coli (LB medium), S. capsulata (MH medium, 30°C; Oxoid), N. aromaticiv-
orans (MH medium), S. yanoikuyae (TSB), and B. burgdorferi (BSK-H medium,
33°C; provided by L. Bockenstedt, Yale School of Medicine, New Haven, CT;
Sigma-Aldrich) were grown in culture medium at 37°C, unless otherwise
noted, until mid- to late-log phase, washed 3× in LPS-free PBS (Invitrogen),
and heat inactivated at 65°C for 45 min. B. burgdorferi were inactivated at
48°C for 30 min. M. tuberculosis were inactivated by radiation (Colorado
State University, Fort Collins, CO). All bacterial preparations, except gram-
negative bacteria, tested negative for LPS by Limulus amebocyte lysate test
(limit of detection 0.03 EU/ml; Associates of Cape Cod).
Lipids, lipid extraction, and MS analysis. Lipids were extracted and ana-
lyzed by thin layer chromatography, as described elsewhere (Tatituri et al.,
2007), or analyzed by high performance liquid chromatography (HPLC)
using a AutoPurification HPLC system coupled to a mass detector with both
ES and APCI capabilities, evaporative light scattering detector, and photo-
diode array detector (Waters). MS analysis, including low-energy CAD MSn
experiments, were conducted on a linear ion-trap (LIT) mass spectrometer
(Finnigan; Thermo Fisher Scientific) with Xcalibur operating system. Lipid
extracts dissolved in chloroform/methanol (1/2) were continuously infused
(2 µl/min) to the ESI source, where the skimmer was set at ground potential,
the electrospray needle was set at 4.5 kV, and temperature of the heated capil-
lary was 300°C. The automatic gain control of the ion trap was set to 2 × 104,
with a maximum injection time of 100 ms. Helium was used as the buffer and
collision gas at a pressure of 103 mbar (0.75 mTorr). The MSn experiments
were performed with an optimized relative collision energy ranging from
18–25%, an activation q value at 0.25, and the activation time at 30–50 ms to
Innate iNKT cell activation during infection | Brigl et al.
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